The present invention pertains to float zone refining of semiconductor material rods and more particularly to a breakapart single turn work coil suitable for manufacturing large diameter monocrystal and polycrystalline semiconductor rods.
Float zone refining or zone melting is a method which has been commercially employed for many years to produce monocrystal semiconductor materials as well as purifying polycrystalline semiconductor materials, such as silicon. These methods produce rod materials ordered having the extremely high purity and crystalline lattice structure without dislocations which are required for making certain electronic devices. The general float zone refining process as disclosed in Keller U.S. Pat. No. 2,922,311. A more recent commercial float zone refining process configuration is disclosed in Campbell U.S. Pat. No. 3,734,695.
Float zone refining of such semiconductor materials is often referred to simply as zone refining and the latter expression is used in this disclosure.
In this process or variations thereof, a molten zone is initially produced at a selected location on, for example, a silicon rod or other semiconductor rod by inductive radio frequency (RF) heating. If the rod is polycrystalline silicon, the molten zone is initially seeded with a small monocrystalline seed crystal. Thereafter, the molten zone is moved along the rod by the production of relative motion between the rod and an adjacent RF induction heating coil encircling the rod. Such relative motion may be provided by well-known mechanical means, such as motor driven shafts, and the traverse rate of the molten zone along the rod will depend upon the required quality of the crystal and upon the power of the RF field produced by the adjacent induction heating coil. In the selection of the above parameters to determine the traverse rate, care must be taken to prevent zone spill over and/or arcing as will be appreciated by those skilled in the art. There may be one pass or a plurality of passes of the molten zone along the length of the rod to achieve a desired ultimate purity, crystalline lattice structure, and/or rod diameter.
Conventional work coils generally are comprised of single turn coils made of a metal, preferably copper, silver or silver plated copper, and having an outside diameter of about 2 to 6 inches (50.8 to 152.5 mm) and an inside diameter of about 1 to 2 inches (25.4 to 50.8 mm), a coil thickness at the outer peripheral of about 0.40 inches (10.16 mm), and a gap spacing between ends of about 0.23 inches (3.05 mm). These sizes are exemplary and may vary depending upon the size of the rod to be refined. Typically, the outside diameter may be about 2 inches (50.8 mm) larger than the rod.
The electronics industry has an ever increasing demand for larger silicon wafers which must, in turn, be cut from larger diameter silicon crystal rods. However, heretofore there has been no known method for multiple pass zone refining and/or adjustable zone refining insitu without breaking the rod from its welded inplace mounting; thus involving waste due to rod breakage.
One of the obvious problems encountered in producing ever increasingly larger silicon crystal rod diameters is found in the use of one turn or single turn coils commonly used in zone processing. Inner diameter of these single turn coils are smaller than the diameter of the feed rod. This geometric relationship poses an ever increasing problem for first passes as rods must be turned around for the final pass in the production of single or monocrystal material. It has not been possible to melt the feed rod up to the carbon handle or weld. At the end of the pass, silicon rods frequently have deficiency problems commonly known as burrs in forms of fingers, and are very difficult to melt. Any effort to melt these protruding burrs completely results in spill overs and arcing, which in turn seriously deteriorates chances for good controllable lattice structure yield. The only way to take the rod from the coil, even after a successful freeze out, that is, a zone process wherein the burrs problem is not a common appearance, is to chip away the material extending beyond the coil opening. Frequently the neck breaks as a result of chipping and another carbon has to be welded thereto. This dismounting process of the rods involves more handling of the zone pass rod and increases chances for fractures of the rod. Frequently such multihandled rods must be scrapped because of the resulting fractures and defects.